1. Technical Field
The present application relates to helicopters and, in particular, to yokes for mechanically coupling helicopter blades to a mast.
2. Description of Related Art
Each blade of the main rotor assembly of a helicopter must be connected to a main support mast, usually by means of a rotor yoke, in a manner allowing several degrees of freedom. Such an interconnection is subjected to high and repeated stresses of both torsional and centrifugal natures, and is therefore an extremely important component of the aircraft. Each blade must be able to rotate about its longitudinal axis to provide pitch control. Each blade must be able to flap in a direction perpendicular to the rotor plane to accommodate vertical loads. In some instances, each blade must be able to pivot within the rotor plane to provide for lead-lag control. The manner in which the blades are secured to the main support mast enables a helicopter to be controlled and maneuvered in flight.
Various structures and mechanisms have been utilized to interconnect the helicopter blades and the support mast. The prior art includes several examples of articulated metal couplings. Such couplings have suffered from the disadvantages of weight, cost, high maintenance requirements, and low useful life. There have been several attempts to eliminate one or more of the articulations in such couplings in order to simplify construction and reduce costs. Some rotor hubs or yokes are pivotally secured to the support mast, and are characterized by a flat plate construction resilient enough to act as a virtual hinge and thereby accommodate flapping of the blades.
More recently, glass fibers and other composite materials have been employed in the fabrication of helicopter rotor system components. For example, a rotor yoke has been constructed by forming a loop from wound filaments with layers of cross plies normal to the central plane of the loop arranged only in the sides thereof. In comparison to a machined metal forging, glass fibers and other composite materials have more favorable fatigue characteristics resulting in longer useful life. In addition, the use of such materials simplifies construction and reduces costs. One of the problems encountered in utilizing such materials in helicopter rotor yokes, however, has been separation of the composite material layers, known as delamination, resulting from excessive mechanical strain. For example, in some situations, a helicopter blade may flap to a greater degree than desired. In such cases, the excessive flapping places stresses on particular portions of the yoke that exceed the interlaminar strength of the composite material in those areas. Moreover, the allowable degree of blade flapping available for a particular helicopter design is limited by the interlaminar strength of the composite material.
There are many designs of helicopter yokes well known in the art; however, considerable shortcomings remain.
While the system of the present application is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the system of the present application to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present application as defined by the appended claims.